In 426, 3,000 hoplites from Ambracia invaded Amphilochian Argos in Acarnania on a gulf of the Ionian Sea and occupied the fort of Olpae.

He joined with the Acarnanian army and set up camp in a ravine opposite Eurylochus, where both sides made preparations for five days.

As the Ambraciot and Peloponnesian army was larger, Demosthenes set up an ambush with 400 hoplites from Acarnania, to be used when the battle began.

Demosthenes surprised them there at night, pretending to be the other Ambraciot army, and killed most of them; the rest fled to the hills or into the sea where they were captured by the 20 Athenian ships.

Although Demosthenes could have easily taken Ambracia, he did not, because his allies feared a strong Athens in that region and so the Acarnanians and Ambraciots signed a 100-year peace treaty with them.